This invention relates to rock bits for drilling oil wells or the like where the cutting action is provided by wear resistant, corrosion resistant tungsten carbide inserts having as a binder phase a cobalt alloy including chromium and nickel.
Oil wells and the like are commonly drilled with rock bits having rotary cones with cemented tungsten carbide inserts. As such a bit is rotated on the bottom of a drill string in a well, the cones rotate and the carbide inserts bear against the rock formation, crushing and chipping the rock for extending the depth of the hole. Typical inserts have a cylindrical body which is pressed into a hole in such a cone and a somewhat blunt converging end that protrudes from the face of the cone. The converging end of the insert may be generally conical, roughly hemispherical, or have a somewhat chisel-like shape.
Another type of bit for drilling rock employs a steel body in which similar tungsten carbide inserts are embedded. Such a rotary percussion bit is hammered against the bottom of the hole for shattering rock and gradually rotated as it drills. Another type of rock bit referred to as a drag bit is simply rotated in the hole with carbide inserts "dragging" across the bottom of the hole for scraping the rock formation. Inserts provided in practice of this invention may be used in either type of rock bit, or in other related devices such as under-reamers.
Since the tungsten carbide inserts are the parts of the rock bit that engage and drill the rock, it is important to minimize wear and breakage of such inserts. Tungsten carbide inserts for rock bits are made by sintering a mixture of tungsten carbide (WC) powder and cobalt to form a dense body with very little porosity. Two important properties of such inserts are wear resistance and toughness. It is desirable to enhance the hardness of an insert where it engages the rock formation and maintain toughness for minimizing breakage of the insert as it is used.
It has been found that an element of wear resistance of rock bit inserts includes resistance to corrosion. Rock bits are commonly used in an environment of drilling mud which may include corrosion inhibitors. However, even so, the drilling mud may have changed pH and chemical composition, such as high amounts of chlorides, which may corrode the inserts as well as the steel of the rock bit. The cobalt binder phase in the cemented tungsten carbide inserts may be leached in either basic or acidic drilling mud, and the cobalt is particularly susceptible to corrosion by chloride containing compositions. It is therefore desirable to enhance the corrosion resistance of the cemented tungsten carbide inserts of a rock bit.
In rock bits designed for a particular type of service, one needs to have an appropriate balance between hardness and toughness. Hard inserts resist wear during drilling. On the other hand, a hard insert may be susceptible to fracture under the impact loads and other abuses necessarily involved in drilling wells. Enhanced toughness is also advantageous, since the part of the insert extending beyond the face of the cone does not need to be as blunt to resist fracture. This means that a longer, more aggressive cutting structure can be employed on a rock bit where fracture toughness is adequate.
In essentially all bits, it is desirable to have high hardness and wear resistance and relatively large insert protrusion. Achievement of these desiderata may, however, be limited by a lack of fracture toughness in the main body of the insert. Thus, it is desirable to have a hard and tough insert with good corrosion resistance.